This invention relates to high solids, large particle size latex compositions particularly useful in corrosion resistant coatings and paint compositions.
Latexes are suspensions of small particles in water. The particles can be natural or synthetic polymers such as rubber, plastics, and acrylics. Illustrative natural and synthetic polymers include latexes which are dispersions, emulsions, suspensions or the like of the following representative polymers: acrylics; vinyl acetate homopolymers or copolymers such as copolymers with acrylates or ethylene; homopolymers and copolymers of styrene including: styrene-butadiene, carboxylated styrene-butadiene, polystyrene and copolymers of styrene and unsaturated acid anhydrides such as maleic anhydrides; polyvinyl alcohol; natural rubber; synthetic rubbers such as butyl rubber, chlorinated or hydrochlorinated rubber latex, and isoprene rubber latex; gums; homopolymers and copolymers of vinyl halides such as vinyl chloride, and vinylidene chloride, with each other or with acrylonitrile or vinyl esters such as vinyl acetate; homopolymers and copolymers of acrylic and methacrylic acid and their esters and derivatives with or without other monomers having C.dbd.C groups; polybutadiene; polyisoprene; olefinic resins such as polyethylene, polypropylene, etc.; carboxylated natural and synthetic latexes and similar polymeric materials. These latexes may also be modified with alkyd materials as is well known in the art.
Latexes are formed through the process of emulsion polymerization. Emulsion polymerization is a free radical addition polymerization process and, as such, retains the gross characteristics of other addition reactions, including the major sequences of initiation, propagation, and termination.
A common guideline for the preparation of stable, non-gelling latexes which are useful as corrosion resistant coatings is that it is generally accepted that the smaller the particle size of the latex the better the film performance in areas such as salt spray resistance and corrosion inhibition. Particle sizes under about 0.1 micron are considered acceptable. Larger particle size latexes typically do not provide the desired corrosion resistance that smaller particle size latexes provide.
There is a direct relationship between the particle size of the latex and the solids content of the latex. The smaller the particle size, the lower the solids content. It is possible to formulate higher solids latexes that have small particle sizes; however, these latexes typically experience extremely high viscosity, tend to gel, and are unstable for their intended purpose. Thus, all commercially available corrosion resistant latexes are small particle size latexes having lower solids contents. A typical corrosion resistant latex contains less than 50% non-volatile materials (NVM), and more than likely contains about 40% NVM.
As stated above, typical latexes suitable for use as and in corrosion resistant coatings and paints have small particle sizes, generally below about 0.1 microns, and have low solids contents generally about 40% NVM. However, a high solids content latex, if stable, would be much more preferred than a lower solids content latex because the high solids content solves many problems associated with paint and coating production. To date, a high solids latex has not been commercially prepared due to the expected disadvantage in corrosion resistance performance from the larger sized particles. Formulating a smaller particle size latex to be higher in solids content typically results in poor rheology, high viscosity, and/or gelling problems, especially when combined with pigments and other standard ingredients found in coatings and paints.
The advantages of a stable higher solids content latex include the ability to incorporate additional ingredients such as reactive pigments and the like using a grind phase which does not contain latex, or only a very small percentage of latex with water. Typically, with lower solids content latexes, a latex/water combination is necessary in the grind phase to produce a coating or paint which has desirable rheological and viscosity properties. The advantage to using water alone in the grind phase is that there is no agglomeration reaction between pigments and latex under the high shear conditions of the grind. Another advantage to using water alone in the grind phase is that the ground pigment can be processed to a smaller particle size resulting in a more stable paint composition. Furthermore, the grind time can be reduced to as little as 25% of the time that would be needed if using a lower solids content latex with a latex/water grind phase. In addition to grinding advantages, the increase in solids content of the latex means a reduction in the amount of water in the latex, resulting in a lowering of the cost of transportation and storage of the latex.
The latexes of the present invention are high solids content large particle size latexes which, when used as, or incorporated into, paint and coating formulations, provide corrosion resistance performance results equivalent to or better than conventional low solids content, small particle size latexes without experiencing the problems associated with larger particle size latexes.